1. Field of the Invention
The present invention relates generally to the field of mechanical ventilation of patients. More specifically, the present invention discloses an open system for providing ventilation in a predetermined flow waveform synchronized to a patient's breathing cycle to augment respiration by a self-breathing patient.
2. Statement of the Problem
Standard mechanical ventilators deliver pressure. There are three classifications of mechanical ventilators that are based upon how they administer pressure ventilation. Negative pressure ventilation requires an apparatus that expands the chest wall, creating levels of sub-atmospheric pressure that draw air or oxygen-enriched ambient gas through the upper airway and into the lungs. Positive pressure ventilation requires that supra-atmospheric pressure is generated and controlled by the device so that air or oxygen-enriched air is pressurized to the degree that it can be forcibly driven through the upper airway and into the lungs. The third method is a combination of positive/negative pressure. The prime example is a high frequency oscillator, where oscillations of negative and positive pressure are produced in the airway in a sinusoidal pattern that is independent of self-breathing efforts and at a rate that exceeds the maximum human respiratory rate by many fold.
Positive pressure ventilators are by far the most frequently used mechanical breathing device. They can be further divided into invasive or noninvasive systems. Invasive systems utilize an endotracheal or tracheostomy tube, with an inflated tracheal cuff that creates an obstruction closing off the upper airway from atmospheric or ambient gas and thus creates a closed system between the positive pressure ventilator and the lungs. This is Closed System Positive Pressure Ventilation (CSPPV). Breath delivery with positive pressure ventilators can be categorized as either pressure-targeted or volume-targeted ventilation. Generation of a specific airway pressure on inspiration and often a different pressure on expiration are pressure-targeted outcomes, or alternatively, a level of pressure is generated to achieve the primary goal of a targeted tidal volume delivered to the lungs (volume-targeted ventilation). The closed system allows a positive pressure breath to be delivered through the inspiratory valve of the device, through the inspiratory limb of the breathing circuit and directly to the lungs without loss of pressure by dissipation of gas into the atmosphere. The delivery of the breath can be forced into the patient independent of the patient's breathing pattern (time triggering) or synchronized with the patient's effort to inhale (pressure or flow triggering), but the patient's normal negative pressure inspiration during self-breathing is lost as it is converted to a positive pressure breath. Peak inspiratory airway pressures of 20 to 30 cm H2O or greater are commonly achieved. The inspiratory valve is open during the patient's entire inspiratory phase. During inspiration the expiratory valve on the expiratory limb of the breathing circuit must remain closed to maintain the pressurized breath. The transition from inspiration to expiration is ultimately governed by the ventilator (breath cycling) and not the patient, because in a closed system, the expiratory valve must open to allow exhalation. During exhalation, the inspiratory valve is closed to prevent retrograde flow of gas back into the machine, which could result in the physiologic terms of rebreathing carbon dioxide or dead space gas, which is dangerous and potentially life-threatening. The expiratory valve is at least partially open to allow the breath to adequately vent into the atmosphere. The pressure at the onset of exhalation with CSPPV approximates the peak inspiratory pressure (e.g., 20 to 30 cm H2O or greater) and dissipates over the expiratory phase as a function of the patient's exhaled gas being allowed to exit through the exhalation valve. Though the expiratory valve or mechanism is often completely open during exhalation, partial closure of the expiratory valve or mechanisms during one or more components of the expiratory phase may achieve a targeted level of expiratory pressure within the lungs while still maintaining adequate exhalation through the valve and acceptable gas exchange. Examples of methods to achieve pressure in the lungs during exhalation due to partial exhalation valve closure during one or more points in exhalation include expiratory retard and positive end expiratory pressure (PEEP). During CSPPV, the expiratory valve is not completely closed during exhalation as life-threatening excessive pressure and suffocation could result. Partial closure of the expiratory valve during end exhalation (PEEP) prevents the decline in airway pressure from ever returning to the 0 cm H2O baseline between exhalation and inhalation. Prescribed PEEP may be 5 to 15 cm H2O or more. On expiration, with CSPPV all the exhaled gas is routed through the expiratory limb of the circuit and is available to the ventilator for analysis. This analysis is required for proper ventilator function and monitoring. More than one CSPPV mode can be administered simultaneously (e.g., intermittent mandatory ventilation with pressure support and positive-end expiratory pressure).
Though CSPPV can be life-saving for patients who are unable to do any negative pressure self-breathing, there are a number of problems with the CSPPV technology. It has been scientifically demonstrated that the pressure generated by positive pressure ventilation can injure the delicate structures of the lungs. This injury can cause significant morbidity and mortality, particularly when CSPPV is superimposed upon acute lung injury from pneumonia or adult respiratory distress syndrome (ARDS). Over time, positive pressures that were once thought to be safe have been determined to cause lung injury. The safe positive pressure threshold that does not cause (or worsen) acute lung injury on some level is presently unknown. There is a scientific trend for documentation of acute lung injury with lower and lower positive pressures as more is learned about the pathophysiology of acute lung injury on organ, tissue, cellular, biochemical and genetic levels. In certain clinical settings positive inspiratory and/or expiratory pressures may impair gas exchange in the lungs. Positive pressure ventilation can cause life-threatening impairment of cardiac output and can cause lung collapse (tension pneumothorax) resulting from barotrauma.
Other issues associated with the endotracheal tube with inflated cuff that is required to maintain pressure with CSPPV can also cause serious injury such as tracheomalasia, tracheoesophageal fistula, tracheal granulation tissue and tracheal stenosis and necrosis. Along with serious injury, the inflated tracheal tube cuff has other consequences. Its use does not allow for communication between the larynx and the upper airway and causes gas to be channeled away from a patient's natural humidification system. This is detrimental to the patient. The inflated cuff prohibits utilization of the vocal cords. Patients are unable to speak causing poor communication between the patient and healthcare providers and family thus impeding proper informed consent and establishment of advanced directives. This absence of speech can cause frustration, anxiety and depression. Bypassing the larynx also impairs cough. Normal closure of the vocal cords allows generation of the glottic blast that facilitates effective cough and clearance of respiratory secretions. Finally, the vocal cords serve as a variable regulator of respiratory flow that fine tunes passage of gas in and out of the lungs to optimize gas exchange.
Use of an inflated cuff also pools upper respiratory secretions above the cuff and, over time, these secretions become contaminated with bacteria. These pooled secretions can leak into the lungs and increase the risk of ventilator-associated pneumonia (VAP). Additionally, inflated tracheal tube cuffs can impair swallow. Though tracheostomy tubes may be more comfortable, endotracheal tubes that pass through the nose or mouth and into the trachea are typically used initially and are very uncomfortable. Tracheostomy tubes require a surgical procedure that can result in a number of complications, including bleeding, infection, barotrauma and airway obstruction.
Liberating patients from CSPPV requires a successful return of the patient to normal negative pressure self-breathing. This has proven to be difficult, particularly when patients have had their breathing controlled and altered by CSPPV for greater than 21 days (prolonged mechanical ventilation, or PMV). In fact, once patients have required CSPPV for greater than 21 days, the wean success rate is only about 50% overall, with a range of 35% to about 60%.
In the past, efforts have been made to allow patients to speak through the upper airway during CSPPV. One method utilizes deflation of the tracheal tube cuff. This open system was not intended for use with standard CSPPV as inadequate ventilation can result. Use of this method may allow a significant portion of the ventilator breath intended for delivery entirely to the patient's lungs to escape through the upper airway. Modes that function based upon targeting a specific inspired tidal volume or pressure and expired pressure can be significantly compromised by the absence of connectivity of the ventilator to closed passages in and out of the lungs and inability of pressures measured in those passages to accurately reflect lung pressure, since pressure can freely dissipate via the upper airway. The volume and characteristics of the exhaled breath passing back to the ventilator in this type of open system will be inaccurate and could compromise the ventilator's ability to use properties such as exhaled tidal volume or airway pressure to evaluate proper ventilator delivery. Unnecessary triggering of alarms such as low pressure or inadequate expired breath volume may occur. Absence of a closed system may result in dys-synchrony between the ventilator and the patient if adequate feedback is not received by the ventilator regarding the patient's breathing efforts thus the ventilator is unable to adequately assess when to deliver the breath or when to terminate the breath. Such ventilator-patient dys-synchrony is known to have deleterious physiologic effects. Some modes such as pressure support ventilation may partially or completely compensate for the leak with the deflated cuff by increasing device driving pressure to maintain the desired pressure delivered to the lungs. However, use of the positive pressure ventilator by this method is still positive pressure ventilation and still pressure-targeted and not flow-targeted. The patient's negative pressure self-breathing is still converted to positive pressure breaths.
When the cuff is deflated, one-way inspiratory valves may be placed in the external CSPPV circuit to prevent gas from flowing back through the machine on exhalation. This is intended to direct expired gas up through the vocal cords to facilitate speech. However, many of the problems noted above that are encountered with using standard closed system CSPPV with an open system still remain. Use of the positive pressure ventilator by this method is still positive pressure ventilation and still pressure-targeted or volume-targeted and not flow-targeted. The patient's negative pressure self-breathing is still converted to positive pressure breaths.
Noninvasive ventilation does not require a tracheal tube with inflated cuff. Bi-level positive pressure ventilation generates a set positive pressure on both inspiration and expiration as the targeted outcomes (pressure-targeted ventilation). An interface such as nasal or nasal/oral mask, full face mask or nasal pillows or similar devices are required to create a barrier between the upper airway and the atmosphere. As with CSPPV, the patient's negative pressure self-breathing is converted to positive pressure breaths. The obstruction created by the interface is great enough that it allows generation of adequate positive pressure to provide all or nearly all of the patient's required breath. The obstruction created by the interface allows lung-distending pressure to be maintained on exhalation which is always less than the pressure on inhalation. The patient both inhales and exhales through the single-limb breathing circuit. As with CSPPV, exhalation of gas back into a circuit maintains proper ventilator function and monitoring. The gas delivery valve connected to the single limb breathing circuit is continuously open during inspiration and expiration to maintain desired inspiratory and expiratory positive pressures. The pressurized breath is triggered either when the patient makes an effort to breathe or time-triggered when a breath is not sensed and a specified time has passed. This prior art is referred to as Substantially Closed System Positive Pressure Ventilation (SCSPPV). Since the interface is usually not completely sealed, there is a leak of pressure and loss of a portion of the gas into the atmosphere. In addition to gas leak through the upper airway, the single limb circuit has an exhalation valve at the distal end that is constantly open during both inhalation and exhalation. The valve remains completely open during inspiration to flush CO2 from the circuit and completely open on expiration to allow the exhaled breath to escape into the atmosphere. Rebreathed CO2 can cause significant morbidity and mortality. Common exhalation valves or mechanisms include Whisper Swivel, Castle Port or NRV devices. Under no circumstances should the valve be occluded. The microprocessor constantly evaluates leaks by comparing pressure in the lumen of the proximal inspiratory circuit to pressure as measured through monitoring tubing in communication with the lumen of the distal end of the circuit. The microprocessor can also compensate by changing the driving pressure to maintain the primary target of delivered pressure (pressure-targeted ventilation) during inspiration and expiration.
With SCSPPV, the absence of a tracheal tube with an inflated cuff avoids a number of the previously described complications of CSPPV. However, the nasal or nasal/oral mask, full face mask or nasal pillows or other devices used with SCSPPV to serve as an interface between the upper airway and atmosphere can be uncomfortable. Skin ulcerations and abrasions can result from tight-fitting masks. Patients in respiratory distress may feel claustrophobic with these devices covering their nose, mouth or entire face. The devices can make speech difficult and eating and swallowing difficult as well. Devices covering the mouth or face can impair the patient's ability to expectorate sputum. Similar to CSPPV, a disadvantage of SCSPPV technology is the requirement for generation of pressure. Depending on the device, peak inspiratory pressures of up to 20 to 40 cm H2O can be generated. The pressurized breath delivery can be uncomfortable. Current medical literature shows that SCSPPV may be neither effective nor tolerated in the extremes of mild and severe respiratory failure. In certain clinical settings positive inspiratory and/or expiratory pressures may impair cardiac output and gas exchange in the lungs. Though not as likely to occur as with CSPPV, acute lung injury and barotrauma may potentially occur when pressures in the high range are delivered. Similar to CSPPV, the use of SCSPPV eliminates normal negative pressure self-breathing. As with CSPPV, successful return to normal negative pressure self-breathing is required for successful discontinuation of SCSPPV.
Transtracheal augmented ventilation (TTAV) is a prior art that is an alternative to positive pressure ventilation. TTAV is not intended to give full ventilatory support like a CSPPV device, but augments the patient's self-breathing by utilizing an open system and delivering a constant and continuous flow of about 8 to 20 L/min of a heated and humidified air and oxygen blend to the lungs during both inspiration and expiration. It is an open system because there is no inflated tracheal cuff and no mask, nasal pillows or other device to create a complete or near complete barrier between the mouth and/or nose and the atmosphere. Because of the nature of the open system, delivered gas can easily escape into the atmosphere and positive pressure is not a targeted outcome. Tidal volume that the patient inspires through the device is not an outcome that can be reliably targeted because of volume loss through the upper airway and variability of volume that the patient inspires through the upper airway during negative pressure self-breathing. In fact, TTAV is only intended for use on patients who are able to do some degree of negative pressure self-breathing. Benefits from augmented ventilation are derived from a defined constant and continuous flow that is superimposed upon the patient's own breathing cycle. Patients can freely inhale room air through the mouth and nose in addition to the gas delivered by the TTAV device. With prior art, air or oxygen enriched air can be delivered directly into the trachea via a transtracheal catheter. The delivery device heats and humidifies the gas to eliminate complications and sequellae from the humidity deficit that would otherwise occur from delivering constant and continuous flows of 8 to 20 L/min of dry cool gas directly into the trachea. There is a single inspiratory circuit with no expiratory circuit or expiratory valve because the patient is free to exhale normally through the nose and mouth. No inspiratory valve is used as a constant and continuous flow is delivered to the patient rather than distinct breaths. Since the constant and continuous flow is superimposed upon the patient's inherent negative pressure self-breathing cycle, synchronization with the patient's breathing is not required. A pressure relief valve prevents over-pressurization within the device in the event of a malfunction or obstruction and an alarm signals the event. Exhalation of gas back into the breathing circuit or into the device is not required to monitor or manage gas delivery during routine operation.
Compared to either low flows used with prior art transtracheal oxygen therapy or mouth breathing without transtracheal flows, potential physiologic benefits of TTAV at a constant continuous flow of 10 L/min include correction of hypoxemia, reduced inspiratory work of breathing, decreased volume of gas the patient must inspire through the upper airway, and improved exercise capacity. The effect of constant continuous TTAV flow above 10 L/min corrects hypoxemia. Since prior studies show that the relationship between flow and response is directly related, one would predict improved response in terms of reduced inspiratory work of breathing, decreased volume of gas the patient must inspire through the upper airway, and improved exercise capacity with flows above 10 L/min. However, the effect on these specific physiologic parameters has not been specifically evaluated. Compared to low flow transtracheal oxygen therapy at 1.5 L/min, potential physiologic benefits of TTAV at a constant and continuous flow of 15 L/min additionally include increased efficiency of breathing, reduced total minute ventilation and reduced end-expiratory lung volume. The effect of constant and continuous TTAV flow above 15 L/min on these physiologic parameters has not been evaluated. Reduced physiologic dead space is seen with low flow transtracheal oxygen (up to 6-8 L/min) as compared to mouth breathing. However, it is not known if constant and continuous flow above 8 L/min with TTAV results in any further reduction in physiologic dead space. TTAV at 10 L/min as a means of augmenting ventilation of patients with chronic respiratory failure during nocturnal home use has been shown to be safe and effective. Furthermore, removal of prolonged mechanical ventilation patients from CSPPV and placement on a constant and continuous TTAV flow from 10 to 15 L/min through a catheter placed within the lumen of a deflated cuff tracheostomy tube has been shown to improve wean success from CSPPV. In this setting, use of the TTAV device and CSPPV device are alternated in an iterative fashion, with a progressive increase of time on TTAV. With the cuff deflated while the patient is on the TTAV device with a constant and continuous flow of 10 L/min, all gas is expired through the glottis and upper airway resulting in the previously described benefits associated with restored speech, more effective cough and return of glottic function as a physiologic variable regulator of respiratory flow. As noted previously, the inflated tracheal cuff prevents these benefits from occurring with CSPPV. It is unknown if constant and continuous TTAV flow above 15 L/min improves effectiveness or wean outcome.
A less than optimal condition associated with TTAV is that a constant and continuous flow is administered throughout the inspiratory and expiratory phases of the respiratory cycle. Each of the potential benefits as described above will likely have different respiratory cycle targeted flow rates and waveforms to achieve maximal beneficial effect in a given patient, and requirements may change with alterations in the clinical status of that individual over time. Additionally, patients with different diseases or disorders may benefit more from certain physiologic effects than from others, and those effects can be influenced by different flows and flow waveforms administered in specific phases (or phase components) of the respiratory cycle. Synchronizing the amount and pattern of flow with specific phases of the breathing cycle or even components of phases of the breathing cycle may markedly influence clinical efficacy. In contrast, constant continuous flows delivered throughout the inspiratory and expiratory phases as seen in the prior art may not be efficacious. For example, a constant and continuous flow of 40 L/min delivered throughout the inspiratory phase of breathing may significantly increase total inspiratory work of breathing rather than reduce it if the specific physiologic effect on the respiratory inspiratory phase and phase transitions as well as the phase components are not considered. With prior TTAV art, that constant and continuous flow of 40 L/min would also be delivered during exhalation. That amount of flow throughout expiration would likely impose a significant expiratory workload causing the patient to forcibly exhale against the constant incoming stream of tracheal gas. This could result in respiratory muscle fatigue and impaired gas exchange. There may be benefit to transiently interrupting flow during certain components of the breathing cycle which could influence clinical efficacy. TTAV with a constant and continuous flow eliminates the potential for improving safety, efficacy and tolerance by the inability of the prior art to target non-constant, potentially non continuous flows with different peak flows and flow patterns that are strategically synchronized with the various phases or components of the phases of a patient's breathing cycle.
Another weakness associated with conventional TTAV systems is that, other than an alarm and pressure relief valve for excessive pressures encountered within the channels of the delivery device and lumen of the circuit, there are no sensors or measurement devices that provide physiologic data that identify phases or components of phases of the patient's negative pressure self-breathing cycle that are designed to regulate breath synchronized, flow-targeted delivery. Conventional TTAV systems do not have microprocessors supporting breath-synchronized, flow-targeted delivery designed to manage patient physiologic data, display the data, trigger alarms for out of range results or incorporate that information into intelligent processing for a feedback loop or servo controlled device response to the physiologic data. Another problem with conventional TTAV systems is that the only clinical implementation to date has been limited to use with a transtracheal catheter.
The prior art also includes ventilation systems based on “flow triggering” a breath that is subsequently supported by CSPPV. As opposed to a drop in circuit/ventilator pressure indirectly indicating a breath effort by a patient, the CSPPV breath is triggered by a presumed effort by the patient to generate inspiratory flow. Though patient inspiratory flow is not directly measured, the breathing effort is presumed because flow inside the expiratory limb is measured to drop to less than the known pass through, or bias flow through the circuit. Flow triggering requires a dual inspiratory/expiratory limb circuit. At some point in the mid to late expiratory phase, the ventilator delivers a predetermined constant flow that circulates through the inspiratory and expiratory limb of the circuit and out through the open expiratory valve. With flow triggering the inspiratory valve or mechanism is partially open in the transition phase between exhalation and inhalation, allowing low flows concurrent with the patient's inspiratory effort to enhance triggering sensitivity of the machine. Flow is measured at both the proximal connection of the inspiratory limb and near the expiratory valve. Any drop in flow is assumed to represent the patient's effort to breathe in gas, and the inspiratory breath is triggered. Though flow through the ventilator circuit may reduce the work the patient has to do to draw in an initial portion of the breath to trigger the ventilator, the delivered breath is still positive pressure generated and is either pressure or volume targeted.
One very different type of CSPPV mode is High Frequency Jet Ventilation (HFJV). A pulsating (non-continuous) jet is delivered via a catheter placed within a tracheal tube with inflated cuff. The pulsing volume is determined by setting a driving pressure in pounds per square inch (e.g., 30 psi) and the set rate is multiples of the patient's breathing rate (e.g. 150 breaths per minute) and not synchronized with the patient's efforts. A second source of gas flow is available from the ventilator circuit that can be drawn into the tracheal tube directly through the patient's breathing efforts or indirectly drawn in by a venturi effect from flow through the device. Gas that passes through the circuit and past the patient's airway must exit through, at minimum, a partially open exhalation valve. Gas exhaled by the patient must also exit via the exhalation valve.
HFJV is different than the present invention for a number of reasons. First, it is a form of Positive Pressure Ventilation (PPV) (i.e., pressure-targeted). Gas is delivered in discreet boluses in a rapid manner not synchronized with the patient respiratory cycle. It is a closed system with the exhalation valve partially or completely open during exhalation. Finally, a second lumen is required to deliver additional flow to the patient.
Another technology that utilizes a catheter placed within a cuff-inflated tracheal tube during concurrent CSPPV is called Tracheal Gas Inflation (TGI). TGI is different than the present invention because, in addition to a delivered CSPPV mode via the tracheal tube, an additional flow of gas is insufflated into the trachea via a catheter in a closed system with the cuff inflated. As with HFJV delivered with CSPPV, a second source of gas is supplied via a second lumen, and gas that exits the patient must exit the exhalation valve. The exhalation valve is partially or completely open during exhalation. With TGI, the second lumen delivers standard CSPPV breaths concurrent with flow through the tracheal catheter. Thus, TGI is a mode delivered in conjunction with one or more CSPPV modes.
3. Solution to the Problem
The present invention provides an open system for flow-targeted ventilation to augment the respiration of a self-breathing patient. A predetermined flow waveform is delivered to the patient's airway in synchronization with the patient's breathing cycle and at a sufficient flow rate to achieve a desired physiologic outcome, such as mitigating pressure in the patient's airway, reducing the patient's work of breathing, flushing carbon dioxide from the patient's airway, and increasing blood oxygenation. The present system can also be integrated into CSPPV and SCSPPV devices. For example, one goal of integrating or combining the present system with PPV in one device is to eliminate the iterative steps of switching the patient back and forth between two separate devices to achieve a needed clinical outcome. Another goal is to improve access of certain patient populations to the medical benefits of the present invention while eliminating the need for capitalization of a separate device. This controls cost, reduces redundancy of delivery devices, increases efficiency, saves space at the patient bedside and improves resource allocation.
The present system is intended to augment ventilation by superimposing continuous, non-constant and, under some conditions, non-continuous flows upon the spontaneous negative-pressure self-breathing of patients. Unlike prior art pressure-targeted or volume-targeted positive pressure ventilation, this invention is flow-targeted because achievement of specific flows and flow waveforms are the targeted outcome. Clinician-defined flows are targeted for specific phases or components of phases of the patient's breathing cycle in order to achieve one or more physiologic improvements. Unlike CSPPV or SCSPPV where positive pressure is either the targeted endpoint or an expected consequence of volume-targeted ventilation, the present invention uses an open system and avoids generation of positive pressures that can cause patient discomfort and injury. Specially designed tubing airway devices maintain an unobstructed interface between the airway and atmosphere. A variety of sensors can be used to detect properties associated with phases of the patient's breathing cycle. A microprocessor receives and processes the data generated by the sensors for intelligent monitoring and regulation of the present system.
In the presence of respiratory distress, the invention mitigates the negative-pressure swings that the patient with respiratory compromise must generate during inspiration and the positive-pressure swings that must be generated during expiration with certain diseases and disorders. These pressure swings result from increased work of breathing (WOB). The present system can mitigate the patient requirement for generating pressure, and can thus mitigate excessive WOB, while still allowing the patient to self-breathe in an open system without the need for CSPPV or SCSPPV. With certain diseases or disorders the patient may benefit by not attempting to inspire through the upper airway at all, but compensate by closing the vocal cords (glottis) and mouth and passively letting the device inflate the lungs at the prescribed flow and flow pattern. Though some pressure is generally encountered, it is not the primary target of the device output and muscular work by the diaphragm and thoraco-abdominal muscles is not required to generate pressure. Thus, the system mitigates a pressure that would be generated by the patient as a result of WOB. The free-breathing patient determines when the transitions between inspiration and expiration occur.